Genes, Pedigrees, and Populations
Investigations In to the World of Genetics
Name_______________________
Objectives
The purpose of this exploration is to . . .
1. review the terms gene, allele, phenotype, genotype, heterozygous, and homozygous, and
practice using them correctly;
2. analyze several human pedigrees to determine the type of inheritance exhibited by the trait; and
3. predict the likelihood that particular individuals in the families will have the trait that is the focus of
the pedigrees.
4. understand the significance of the Hardy-Weinberg law in predicting that allele frequencies
remain the same from generation to generation;
5. determine gene frequencies under the conditions of codominance and recessive inheritance; and
6. explore the impact of effects such as selection and migration on allele frequencies.
Part 1 Genes and You.
The example we will explore in class today, PTC taste testing, is an example of alleles that exhibit
dominance/recessiveness. Pedigree studies of human families have indicated that the ability to taste
phenylthiocarbamide (PTC) is due to the presence of a dominant allele (T) and that the inability to
taste the chemical occurs in homozygous recessive individuals (tt). There are two phenotypes
(tasters and non-tasters), but three genotypes (TT, Tt, and tt).
Procedure Part 1
You will be given a small piece of filter paper that has been lightly saturated with PTC. Before you
place the PTC paper on your tongue, take a piece of untreated paper and taste it. This wills serve as
a control so that you can readily determine the difference between the taste of the paper and the
taste of PTC.
Fill out the questions for “Part 1” on your worksheet now.
Most hereditary traits in humans and other species are controlled by multiple
genes with multiple alleles that utilize complex gene interactions to result in the
organisms phenotype. The following human characteristics are controlled by a
single gene with two alleles.
Determine your phenotype for each of the following traits. Also determine your
genotype if possible. If you have the dominant phenotype, you will not know
whether you are heterozygous or homozygous.
What procedures could you use to determine whether or not you are
homozygous? Other traits that are easily observed and controlled by a single
gene are listed below. Try to identify both your genotype and phenotype for
each of these.
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Trait
1. Bent Pinky (B,b) Dominant allele causes distal segment of fifth finger to bend distinctly inward
toward fourth (ring) finger
2. Tongue Rolling (R,r) Dominant allele in
heterozygous or homozygous condition
allows people to roll their tongues into a tubelike shape.
3. Blue eyes (E,e) Homozygous recessive individuals lack pigment in the iris, others have iris
pigment color determined by other genes.
4.Widows Peak (W,w) Dominant allele causes
individuals to have V-shaped front hairline.
5. Thumb crossing (T,t) In relaxed interlocking of fingers, the left thumb over the right indicates the
dominant allele is present; homozygous recessives naturally place right thumb over left.
6. Attached Earlobe (A,a) Dominant allele causes
individuals to have detached ear lobes.
7. Hitchhiker Thumb (H,h) Homozygous recessives can bend the distal joint of the thumb backward to
nearly a 90 angle.
8. Red-green colorblind (C,c)Sex linked trait (on the X chromosome). Recessive allele on the X
chromosome causes the color blindness.
9. Hairy Ears (M,m) Sex linked trait (on the Y chromosome). Dominant allele causes individual to
have hair growing on the outside edge of the ears.
10. Pattern Baldness (P,p) A sex influenced trait. Dominant allele causes premature loss of hair on
top and front of head in heterozygous or homozygous dominant males.
11. Finger length (F,f) A sex influenced trait that involves the length of the index finger relative to the
ring finger. In males, the allele for short index finger is dominant; in females it is recessive
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Part 2 Human Pedigree Analysis
Introduction
The inheritance of human traits is typically determined using a technique called
pedigree analysis. Pedigrees are “family trees” that show which individuals in a
family exhibit a particular trait and how they are related to other affected and nonaffected family members. This information, plus a basic understanding of Mendelian
genetics, is used to make hypotheses about the inheritance of the trait and to make
predictions about the probability that a child will have the trait. Genetic counselors
use pedigree analysis, among other skills, in their work.
In a pedigree, circles represent females, while squares represent males. A diamond
indicates that the sex of the individual is unknown. Shaded symbols indicated that
the individual exhibits the phenotype under consideration. For example, in a
pedigree that examines the inheritance of sickle cell disease in a family, a shaded
symbol indicates an individual with this disease. A horizontal line connects parents;
the vertical line from them leads to their offspring. All of their offspring are “sibs” and
are connected by a horizontal “sibship line.” They are placed from left to right in
order of birth. Connected diagonal lines indicate twins.
Analyzing inheritance patterns revealed in pedigrees
A pedigree is typically used to determine the type of inheritance involved for a trait
and the probability that a specific family member will exhibit the trait in question.
Inherited traits due to a single gene may be inherited in one of four general ways:

Autosomal recessive inheritance. The trait is due to a gene on an
autosomal chromosome (chromosomes 1 to 22) and is expressed only in the
homozygous recessive condition. Pedigree “tip-offs” for this kind of
inheritance: both males and females are affected; unaffected parents have
an affected child.

Autosomal dominant inheritance. The trait is due to a gene on an
autosomal chromosome and is expressed whenever the gene is present
(either homozygous dominant or heterozygous condition). As a rule of thumb,
most individuals with a dominant trait are heterozygous (unless information in
the pedigree reveals otherwise). Pedigree “tip-offs:” both males and females
are affected; affect individuals have at least one parent who also exhibits the
trait.

X-linked inheritance (also known as “sex-linked inheritance”). The trait is
due to a gene on the X chromosome. Most traits of this type are recessive
and are expressed in males who receive an X chromosome carrying the gene
from their mothers. Traits of this type are expressed in females only if they
are homozygous, that is, if they received X chromosomes carrying the gene
from both parents. Thus, males exhibit the trait far more often than females—
in fact, most pedigrees of this type will show only males with the trait.
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
Y-linked inheritance. This type of inheritance is due to a gene carried on the
Y chromosome and, except for genes that cause an individual to be male, is
very rare. Pedigrees for traits with this type of inheritance will show that only
males exhibit the trait and that all affected males have fathers who also have
the trait.
In this lab, you will work in pairs to analyze several pedigrees. This is an opportunity
for you to review several genetics terms and concepts. Keep in mind that “real life”
situations are often more ambiguous that those described above. For example,
penetrance is a phenomenon that makes pedigree interpretation difficult. It refers to
the likelihood that an individual who has the genotype for a particular trait will
actually have the phenotype for the trait. If a dominant trait has 100 percent
penetrance, then every individual who has a dominant allele will exhibit the trait
associated with it. If a dominant trait has 90 percent penetrance, then 90 percent of
the individuals who have the allele will have the corresponding phenotype, but 10
percent of them will not show the trait (it does not “penetrate” in them).
Procedure Part 2
1. Obtain a set of pedigrees from your instructor.
2. In pairs, discuss the pedigrees and answer the following questions for each one:
a. Is the gene associated with the trait on an autosomal chromosome or the X
chromosome? Explain.
b. Is the trait conferred by a dominant or recessive gene? Explain.
c. What is the probability that individual A will have the trait? Explain your
reasoning.
d. What is the probability that individual B will have the trait? Explain your
reasoning.
Fill out the table for “Part 2” on your worksheet now.
Part 3. Genes in Populations, the Hardy-Weinberg Equilibrium
Introduction
The English mathematician G.H. Hardy, and the German physician, W. Weinberg
formulated the basic law of population genetics independently in 1908. It is called
the Hardy-Weinberg equilibrium in their honor. This principle allows population
geneticists to calculate the frequency of alleles of a gene in a randomly interbreeding
group of plants or animals. Hardy and Weinberg demonstrated that in such natural
populations allele frequencies reach equilibrium (that is, the frequencies of the
different alleles of a gene remain constant generation after generation), provided
that no disturbing effects occur such as those caused by mutation, migration,
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selection pressure, non-random mating, or random genetic drift (which occurs
in small populations).
For example, consider a gene that has two alleles, A and a. Let p equal the
frequency, or percent, of the A alleles in a population and q equal the frequency, or
percent, of the a allele. Then, p + q = 1. If random mating occurs, the population
should consist of p2 AA individuals, 2pq Aa individuals and q2 aa individuals (see the
Punnett square below).
Alleles carried by sperm
in the population
Alleles carried by
ova in population p = A
q=a
p=A
q=a
p2 = AA
pq = Aa
pq = Aa
q2 = aa
If the table above is confusing to you, try substituting values for p and q. For
example, instead of p sperm/ova with the A allele and q with the a allele, substitute
0.7 sperm/ova with A and 0.3 with a. Then, what percent of offspring with the
genotype aa would you expect?
The generation offspring represented in the Punnett square will consist of p 2 (AA) +
2pq (Aa) + q2 (aa) individuals. According to the Hardy-Weinberg law, the next
generation should consist of exactly the same frequencies of each genotype.
If a population is in Hardy-Weinberg equilibrium, it will stay in equilibrium, generation
after generation. Neither the allele frequencies nor the genotype frequencies will
change as long as the following apply: random mating occurs; there is no selection
for particular genotypes; there is no migration into or out of the population; and the
population is relatively large.
Determining gene frequencies when alleles exhibit dominance/recessiveness
When dominance and recessiveness affect a pair of alleles, it is impossible to detect
all three genotypes by their phenotypes and to calculate gene frequencies directly.
However, if you assume the population to be in Hardy-Weinberg equilibrium, you
can estimate gene frequencies using the Hardy-Weinberg equations.
You know that q2 represents that frequency of homozygous recessive individuals.
The square root of this frequency is q; knowing q you can determine p using the p +
q = 1 equation. Once you have determined the values for p and q, you can calculate
the frequencies of homozygous dominant individuals (p2) and heterozygous
individuals (2pq).
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For example, if you know that 16 percent of a particular population is made up of Rhnegative people (dd):
q2
q
=
=
0.16;
0.40
=
frequency of the recessive allele d.
Then,
p = 1 – q = 1 – 0.4 = 0.60
=
frequency of the dominant allele D.
The 84 percent of the population that is Rh-positive can be divided as follows:
2pq = 2 x 0.6 x 0.4 =
0.48
=
48% of the population expected to
be heterozygous Dd; and
p2 = 0.6 x 0.6 = 0.36 = 36% of the population expected to be homozygous
DD.
Procedure Part 3
Using the information gained about PTC tasting in Part 1 complete the table for your
lab section.
Fill out the questions for “Part 3” on your worksheet now.
Determining gene frequencies when alleles exhibit codominance
The Hardy-Weinberg equations are not needed to calculate gene frequencies in the
case of codominant inheritance. In these cases, gene frequencies can be
determined directly from the phenotypes of the individuals involved. Suppose you
wish to determine the frequencies of the alleles A (for normal hemoglobin) and S (for
sickle cell hemoglobin) in an African American population consisting of 343 AA
(normal), 294 AS (sickle cell trait), and 63 SS (sickle cell disease) individuals.
(Heterozygous individuals—those with sickle cell trait—can be determined by
microscopic examination of their red blood cells; under conditions that reduce the
oxygen tension in the environment of the cells, some proportion of their cells will
exhibit the characteristic sickle shape. These conditions do not ordinarily occur
within the body of a heterozygous individual, although heterozygotes may
experience some sickle cell disease symptoms at high altitudes.)
Suppose you want to know whether the population is in Hardy-Weinberg equilibrium.
There are 1400 alleles for the hemoglobin gene in the total population of 700 people
(each individual carries two alleles of the gene). The number of A alleles is 980:
343 people have two A alleles and 294 have one A allele. The frequency of A is 0.7:
(343 x 2) + 294
686 + 294
980
1400
=
1400
=
1400 = 0.7
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Similarly, the frequency of S is 0.3:
(63 x 2) + 294
1400
=
126 + 294
1400
=
420
1400
= 0.3
If this population is in Hardy-Weinberg equilibrium, then the frequency of
homozygous dominant (AA) individuals is 0.7 (the probability of having one A allele)
x 0.7 (the probability of having a second A allele) = 0.49. Thus, you would expect
that 49 percent of the population would have the AA genotype. Similarly, 2pq, or 2 x
0.7 x 0.3 = 0.42, or 42 percent of the population should be heterozygous. You would
expect only 9 percent (0.3 x 0.3 = 0.09) of the population to have sickle cell disease
(SS). Applying these percentages to the population of 700 individuals reveals that it
is in perfect Hardy-Weinberg equilibrium:
49% x 700 = 343
42% x 700 = 294
9% x 700 = 63
Part 4. Revisiting the Micro-evolutionary Process and the
Hardy-Weinberg Equilibrium
In 1950 Atlantic Puffins were introduced to two islands that are
763 miles apart, San Carlos and Nola Rei. Every 6 years
more puffins were released on the San Carlos, but not on Nola
Rei due to lack of funding. No puffins were observed moving
between islands. In 2000 researchers noticed that nearly all the
puffins on Nola Rei had completely orange beaks instead of the
striped ones puffins usually have, both populations appear to
be stable with 75 breeding pairs
Using the information above and the H-W equations, complete
the information on the worksheet.
Fill out the questions for “Part 4” on your worksheet now.
Part 5. Incomplete dominance and Co-dominant and models
Cats generally have long tails controlled but one gene in a
incomplete dominance model where
LL are long-tailed,
Ll are bobtails, and
ll are stump-tailed cats.
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Blood types in humans follow a co-dominat model. There are three
alleles involved yielding 4 phenotypes
Using the information above, complete the information on the worksheet.
Fill out the questions for “Part 5” on your worksheet now.
Part 6 Multiple Genes in Action
The following gel electrophoresis is for Labrador retrievers. Coat color arises
through interactions among alleles of two gene pairs. One gene pair codes for
melanin production (B) the dominant form allows production of black pigment and
the recessive form(b)allows for brown or chocolate color. The
other gene codes for melanin deposition (E) in order to place
pigment in the hair the dominant form must be present, if only the
recessive allele (e) is present the dog will have yellow coat color.
Procedure Part 5
Using the information above, complete the information on the
worksheet.
Fill out the questions for “Part 6” on your worksheet now.
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BIO 101 GENETICS
Name _____________________
Part 1 Genes and You
1. Are you a taster of PTC?
If you are a taster, what did it taste like? Was it sweet, sour, salty, bitter, etc.?
2. You now know your phenotype. Do you know your genotype?
a. If no, why not?
b. If no, how might you be able to determine what your genotype is?
3. Fill out the chart below for yourself and one or more people.
Trait
Your
Traits
Other
Phenotype
Possible Genotype Phenotype
Person
Possible Genotype
Bent
Pinky
Tongue
rolling
Blue Eyes
Widows
Peak
Thumb
Crossing
Attached
Earlobe
Hitchhiker
Thumb
Redgreen
Colorblind
Hairy
Ears
Pattern
Baldness
Finger
length
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Part 2 Human Pedigree Analysis
Complete the table below for the traits exhibited in the six pedigrees you analyzed in
class.
Autosomal or
Dominant or
Probability A
Probability B
Pedigree X-linked?
Recessive?
will have trait
will have trait
1
2
3
4
5
6
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Part 3. Genes in Populations, the Hardy-Weinberg Equilibrium
1. Record your phenotype on the table on the board or overhead projector. After all
the data for the class is recorded, copy the total call data below:
Phenotype
Number
Frequency (Percent)
Taster
Non-taster
TOTAL
2.Calculate the frequency of the recessive allele, t, in your class by applying the
Hardy-Weinberg equations to the class data:
3. Use the equations to determine the frequency of the T allele:
4. According to the Hardy-Weinberg equations, the frequency of TT individuals is:
5. . . . and the frequency of Tt individuals is:
Part 4. Microevloutionary Process and the Hardy Weinberg Equilibrium
1. As you may expect, the conditions for Hardy-Weinberg equilibrium do not always
exist five factors disturb genetic equilibrium in natural populations they are:
1. ________________________________________________________
2. ________________________________________________________
3. ________________________________________________________
4. ________________________________________________________
5.________________________________________________________
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2. If we assume a single dominant gene controls that beak-stripping allele (S) and
that the recessive allele is (s) causes a non-striped completely orange beak. Given
the following data calculate the Hardy-Weinberg Equilibrium:
Phenotype
Stripped
Beaks
Orange
Beaks
Total
Initial
Releases
80
0
80
San
Carlos
149
1
150
Nola
Rei
13
137
150
p freq
q freq
Part 5. Incomplete and Co-dominant and models
1.What are the possible outcomes and probabilities from the following cross?
Ll x Ll (Please show your work.)
2.What are the possible outcomes and probabilities from the following cross?
Ll x ll(Please show your work.)
3.What are the possible outcomes and probabilities from the following cross
IAi x IBi (Please show your work.)
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3.What are the possible outcomes and probabilities from the following cross
IAi IB x iii (Please show your work.)
4. Could a marriage between the individuals list in question #3 have a Type-O child?
Part 6. Multiple Genes in Action
1.What is the probability of getting a yellow lab from the cross listed below
EEBb x EeBb (Please show your work.)
2. What is the probability of getting a chocolate lab from the cross listed below
EeBb x eeBB (Please show your work.)
3. What is the probability of getting a black lab from the cross listed below
Eebb x eeBB (Please show your work.)
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NAMES
DATE
Analysis Question 1
Examine the pedigree below for a rare trait found in a particular family:
Gen I
Gen II
Gen III
Analyze this pedigree below Give specifics on the type of inheritance and How you
know, use genetically appropriate language.
Is it dominant or recessive? ______________________________
How you know?
Is it autosomal or sex-linked? ____________________________
How you know?
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Analysis Question 2
The Hardy-Weinberg law applies not only to human
populations, but also to plant populations and other
animal populations. T and N represent two alleles
of the gene for venom found in a population of
gaboon vipers. A study of 2000 vipers from the
population revealed the following: 100 were
genotype NN (non-venomous), 800 were genotype
TN (mildly venomous), and 1100 were TT (deadly
venomous).
Analyze this group of gaboon vipers, from a population genetics perspective.
(Hint your need to use the codomiant model) Show your work.
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